Particulates or particulate matter from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). In order to reduce such kind of particulates, it has been carried out to incorporate a particulate filter in an exhaust pipe through which an exhaust gas flows.
Such kind of particulate filter has a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets are plugged at outlets thereof. Thus, only the exhaust gas passing through thin porous compartment walls is discharged downstream.
The particulates in the exhaust gas, which are captured and accumulated on inner surfaces of the thin porous walls, require to be burned off for regeneration of the particulate filter before exhaust resistance increases due to clogging. However, the exhaust gas from the diesel engine in a normal engine operation status rarely has a chance to reach a temperature level at which the particulates ignite by themselves.
In order to overcome this, an oxidation catalyst comprising, for example, alumina carrying platinum and added with an appropriate amount of rare earth element such as cerium is integrally carried by a particulate filter. The oxidation catalyst facilitates an oxidation reaction of particulates captured by the particulate filter to lower the ignition temperature, so that the particulates may be burned off even at an exhaust temperature level lower than ever before.
However, even in such a case, a captured amount of the particulates may exceed a treated amount in an operation area having a lower exhaust temperature level. Continued operation at such lower exhaust temperature level may hinder sufficient regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the particulate filter. Thus, when an amount of accumulated particulates has increased, it is necessary to forcibly heat the particulate filter by filter regenerating means so as to burn off the captured particulates.
Such filter regenerating means has been developed which is, for example, an oxidation catalyst disposed in front of the particulate filter with fuel being added upstream of the oxidation catalyst to perform heating by use of oxidation reaction heat on the oxidation catalyst, or heating means such as a burner or an electric heater disposed in front of the particulate filter.
Measures for forcibly regenerating such particulate filter are also dealt with in Patent Literatures 1 and 2 by the applicant of the present invention.